On May 31, 2014, in the small town of Waukesha, Wisconsin, two seemingly harmless bespectacled 12-year-old girls stabbed a classmate 19 times. The attack was inspired by the Slender Man or Slenderman, an imaginary faceless specter, mythologized on the internet via user generated fan fiction and video games. The attack and the internet phenomena itself are the subjects of a new HBO documentary, Beware the Slenderman, airing next year on January 23rd, and whose first trailer just went live on YouTube.

The girls, Anissa Weier and Morgan Geyser, were friends and classmates with their victim. Following a sleepover party, they lured her into a nearby wooded area, where one of them held her down while the other stabbed her 19 times in the in the arms, legs, and abdomen. After telling the girl they were going to get help and leaving the scene of the crime, the victim crawled to a nearby road, where she was discovered by a passing cyclist. She suffered organ failure and blood loss, though survived the ordeal and eventually made a full recovery.

After being arrested and charged with first-degree intentional homicide, Weier and Geyser told police they had tried to kill the girl to appease the Slender Man. The Slender Man was created as a piece of fan art and fiction in the forums section of the comedy website Something Awful in 2009. The tall faceless ghoul has inspired countless memes and works of “creepypasta,” horror-related fan fiction shared online. The girls had become obsessed with Slender Man, and thought he was real. By killing their friend, they would become his servants, which would prevent him from harming their families. Since their arrest, defense lawyers have tried to argue that the girls were unfit to stand trial due to mental incompetence, however, last August an appeals court ruled the girls could stand trial as adults.